For a molten iron production process using a rotary hearth furnace (moving hearth reducing furnace) connected to a smelting reducing furnace (iron bath-type melting furnace), Kaneko et al. have suggested a process for producing molten iron by a so-called smelting reduction method including preliminarily reducing, in a rotary hearth furnace, compacts (carbonaceous material-containing agglomerates) made by agglomerating fine powder ore (powdery iron oxide source) and fine powder carbonaceous material (powdery carbonaceous reductant); continuously charging, with a charge shoot or the like, the preliminarily reduced compacts (solid reduced iron) into the smelting reducing furnace (iron bath-type melting furnace) in which a molten metal bath is generated; burning a fuel and a fine powder carbonaceous material (carbonaceous material) supplied as a reductant under excess oxygen to conduct secondary combustion while smelting and reducing the preliminarily reduced compacts to obtain a molten iron bath; and introducing high-temperature discharge gas from the smelting reducing furnace into the rotary hearth furnace to effectively utilize the discharge gas (refer to Patent Document 1).
However, the smelting reducing method described above has many drawbacks as described below that remain unresolved and has not yet put into practical application. Namely, the discharge gas from the smelting reducing furnace contains large quantities of dust which adheres and deposits on the furnace wall or the like of the rotary hearth furnace, the purity of the produced iron tends to be instable due to fluctuation of the calorific value or the like of the discharge gas caused by fluctuation of heat in the smelting reducing furnace, and incorporation of a high FeO slag in the dust contained in the discharge gas would erode the hearth refractory.
In this respect, the inventors of the present invention have developed a process for producing molten iron in a rotary hearth furnace (moving hearth reducing furnace) connected to a melting furnace (iron bath-type melting furnace), the process including thermally reducing compacts containing iron oxides and a carbonaceous reductant on a rotary hearth to produce solid reduced iron with a metallization ratio of 60% or more, feeding the resultant solid reduced iron to the melting furnace, and melting the solid reduced iron while burning the carbonaceous material supplied as a fuel with oxygen and controlling the secondary combustion ratio in the melting furnace to 40% or less so as to obtain a molten iron bath. The inventors have also suggested that the thermal efficiency and the like of the process as a whole can be improved by supplying all or part of the carbonaceous material, which is used as a fuel to be fed to the melting furnace, on a hearth so that the carbonaceous material is used as a bedding carbonaceous material (refer to Patent Document 2).
According to this process, erosion of the furnace refractory can be suppressed as much as possible and molten iron with high iron purity can be produced with high production efficiency. However, as in typical practice, when the bedding carbonaceous material (char) whose weight is reduced by carbonization in the rotary hearth furnace is continuously fed into the melting furnace from above the melting furnace, part of the bedding carbonaceous material scatters into a space (free space) in the upper part of the melting furnace before reaching the surface of the molten bath, and becomes dispersed in the discharge gas from the melting furnace, thereby leading to a decrease in yield of the carbonaceous material. Thus, there has been room for improvements.
On the other hand, applicant of the present invention has suggested a process for producing metallic iron by spreading a powdery bedding carbonaceous material for atmospheric control on a hearth of a rotary hearth furnace, feeding a raw material containing an iron oxide-containing substance and a carbonaceous reductant, and reducing and melting the raw material under heating in the furnace, in which the bedding carbonaceous material discharged from the rotary hearth furnace is recycled and used in the rotary hearth furnace to prevent the powdery bedding carbonaceous material from accreting into a cracker-like material (refer to Patent Document 3).
However, the above-mentioned process does not involve a melting furnace and produces metallic iron by using only the rotary hearth furnace. Thus, the process is inherently free from the problem of scattering of the bedding carbonaceous material into discharge gas from the melting furnace, and cannot be directly applied to a molten iron production process involving a combination of a rotary hearth furnace and a melting furnace.    Patent Document 1: Japanese Examined Patent Application Publication No. 3-60883 (Claims, FIG. 2, etc.)    Patent Document 2: Japanese Unexamined Patent Application Publication No. 2004-176170 (Claims and [0039] to [0042])    Patent Document 3: Japanese Unexamined Patent Application Publication No. 2003-213312 (Claims, FIG. 1, etc.)